A liquid crystal display has widely spread as a display for a note type personal computer, a portable electronic gadget, a portable television receiver and the like because of its features of low voltage drive, light weight, low cost and the like. Among them, a TFT (thin film transistor)-LCD (liquid crystal display) using a liquid crystal of a TN (twisted nematic) type has widely been getting used as a high performance display which is a substitution of a cathode ray tube since the liquid crystal display has features of high precision, high image quality and high speed response. However, when the liquid crystal display is compared with a cathode ray tube, a fatal fault of a display using a TN type liquid crystal is that there is a viewing angle dependence of a display. The viewing angle dependence is a problem to decrease in a display performance (contrast and degradation) when a display is seen along an oblique direction. While this is caused by use of a liquid crystal with an optical anisotropy or a polarizing plate and an LCD generally has such a fault, a problem relating to a viewing angle dependence is severer in a TN-TFT-LCD than in other types of display method since the TN-TFT-LCD has a target specialized in high quality display. For example, in a liquid crystal television receiver which is one of important applications of TFT-LCD, a small change in color or lightness by a viewing angle is sensed by a user as uncomfortable. When a larger area is to be realized, there arises another problem that the same levels of display cannot be obtained at a center and in the periphery of a picture plane since a viewing angle is different at the positions on the picture plane.
In a LCD which is normally white, light is transmitted through in a condition of non-selection and a display is obtained by intercepting light which is caused under application of a selective voltage. A viewing angle dependence of a liquid crystal display is caused by an orientational order of a liquid crystal in a TN liquid crystal cell for drive and especially by an orientational state of a liquid crystal in a light intercepting condition in a greater degree. Therefore, in order to compensate a viewing angle, it is only required to pay attention to an orientational state of liquid crystal when a selective voltage is applied (in a black display) and all that is required for a compensating plate is that it has an effect to extinguish a refractive index anisotropy of an orientational state of a liquid crystal. Features of an orientational order of a liquid crystal cell for drive in a black display are the following two:
1) While liquid crystal molecules are oriented approximately along a perpendicular direction to an electrode substrate, the molecules assume a near-homogeneous orientational order in the vicinity of the substrate due to an anchoring effect. That is, a director of a liquid crystal is not uniform but an inclination to a cell substrate continuously changes along a thickness direction. PA0 2) An influence of a twisted orientational order remains because of anchoring of a substrate. If an applied voltage in a black display is extremely higher, almost all the liquid crystal molecules can be orientated along a perpendicular direction to the substrate and a twist in a twisted orientational order can be extinguished. However, the twisted orientational order cannot perfectly be extinguished in a practical range of an applied voltage.
Conventionally, a compensating film to improve a viewing angle with an attention paid to an orientational state of a liquid crystal cell for drive has been disclosed in, for example, JP 8-50206 A, JP 8-334621 A, JP 9-61624 A, JP 9-26572 A and the like. These publications have paid attention to a fact that a liquid crystal molecule in the liquid crystal is in the shape of a column and a nature of being a negative uniaxial material has been utilized in common among them.
In the JP 8-50206 A and JP 8-34621 A, from the view point of 1) as above mentioned, a hybrid orientational order of a discotic liquid crystal is utilized as a compensating film and a compensating film whose orientational order is fixed has been proposed. However, an influence of a twist in 2) as mentioned above cannot be neglected and thereby the performance of the compensating film has also a limitation.
A teaching included in the JP 9-61624 A is to make a twist effect exercised in a false manner by stacking a plurality of films. However, while a liquid crystal in a liquid crystal cell for drive is continuously twisted and thereby a inclination of a director also changes in a continuous manner, a compensating film of a multi-layer type obtained by means of stacking necessarily has discontinuity in not only a twist but inclination of a director and thereby there is present a limitation on performance. Besides, there are many problems from a view point of production since a process for forming a multi-layer structure is complicated.
The JP 9-26572 A discloses a compensating film made of a discotic liquid crystal having a twisted orientation order. This film has a twisted orientational order in which a magnitude of a projected vector on a film plane of a director of a discotic liquid crystal changes along a thickness direction. In the case of this film, the features recited in 1) and 2) as mentioned above are basically satisfied and therefore a compensating effect which have not conventionally been available can be estimated to be exercised, as clearly seen form description of the publication.
When a viewing angle compensation is achieved by use of a compensating film as mentioned above, optimization of a liquid crystal cell and a compensating film is necessary in order to obtain a sufficient compensating effect. If the optimization is not effected, a compensating effect cannot sufficiently be exercised even if the compensating film is ideal so as to satisfy the features of the 1) and 2) as mentioned above. That is, when a liquid crystal cell for drive and a compensating film are used without optimization on a structure of an orientational order of a liquid crystal in each of the liquid crystal cell for drive and the compensating film, and a relative arrangement of the liquid crystal cell to the compensating film, there is a fear that a display performance of a LCD is further deteriorated.
As mentioned above, while progress in development of an LCD of a TN type which has a feature of a broad viewing angle has been conspicuous in these days, a liquid crystal display device having a sufficient performance has not been obtained yet.